This invention relates to an immproved process for air-laying fibers to produce a wide variety of air-laid non-woven webs, and to an apparatus upon which such a process may be preformed. Preferably, the webs produced by the process and apparatus of the invention comprise a blend of long and short fibers; i.e., textile length and papermaking fibers, with the fibers of the webs being randomly oriented.
Fibers are usually classified according to length, with relatively long or textile length fibers being longer than about 1/4 inch and generally between 1/2 and 21/2 inches in length. The term "long fibers" as used herein, refers to textile fibers having a length greater than one-fourth inch, and the fibers may be of natural or synthetic origin. The term "short fibers," as used herein, refers to papermaking fibers, such as wood pulp fibers or cotton linters having a length less than about 1/4 inch. While it is recognized that short fibers are usually substantially less costly than long fibers, it is also recognized in many instances that it is desirable to strengthen a short fiber product by including a blend of long fibers therein.
Nonwoven materials are structures which in general consist of an assemblage or web of fibers, joined randomly or systematically by mechanical, chemical or other means. These materials are well known in the art, having gained considerable prominence within the last twenty years or so in the consumer market, the industrial commercial market and the hospital field. For example, nonwoven materials are becoming increasingly important in the textile and related fields, one reason being because of their low cost of manufacture for a given yardage as compared to the cost of more conventional textile fabrics made by weaving, knitting or felting. Typical of their use is hospital caps, dental bibs, eye pads, dress shields, shoe liners, shoulder pads, skirts, hand towels, handkerchiefs, tapes, bags, table napkins, curtains, draperies, etc. Generally speaking, nonwoven materials are available today in a wide range of fabric weights of from as little as about 100 grains per square yard to as much as about 4,000 grains or more per square yard.
Nonwoven materials are basically one of two types -- oriented or random webs. As the name implies, oriented webs have the major proportion of the fibers aligned predominantly in one direction, generally the "machine" or long direction (MD) of the fibrous web so that the properties of the resulting web are asymmetrical or aniosotropic -- i.e. conventionally the tensile strengths in the machine direction are generally approximately eight or more times higher than in the cross direction (CD); while on the other hand, random fibrous nonwoven webs do not have the fibers lying predominantly in any direction so that the resulting web is more balanced or isotropic -- e.g. the tensile strengths in both the machine and the cross direction are approximately the same. As will be readily appreciated, the uses of oriented nonwoven webs are quite restricted as compared to random webs in that their principle strength lies only in one direction making them unsuitable where a product must have good strength characteristics in all directions.
Many different processes and apparatus are known in the art for producing nonwoven webs; briefly summarized, they may be classified as (1) mechanical techniques (e.g. by carding, garnetting, filament winding), (2) extrusion techniques (e.g. filament extrusion), (3) wet laying techniques (e.g. inclined wire paper apparatus, cylinder paper apparatus, etc.) and (4) air-laying techniques. This invention concerns improvements in the latter classification -- i.e. the air-laying techniques, to produce improved random air-laid nonwoven materials.
In brief summary, conventional air-laying techniques for producing nonwoven materials involve opening of fibers from a compressed state, dispersing the fibers in a single high velocity air stream and subsequent condensing (i.e. depositing) of the fibers onto a perforated cylinder or wire screen or belt to produce a web. Thereafter, the web is generally post-treated to provide the required degree of coherency by one or more well known steps, e.g. mechanical or chemical bonding procedures.
In general, air-laying techniques of producing nonwoven webs have several advantages over other types of known web processes in its ability to produce a wide variation of lengths and fineness of webs with a wide range of fabric weights, and as well to permit the use of short fibers for different types of products.
Notwithstanding the advantages of air-laying procedures, the present state of technology for producing random nonwoven webs, insofar as their production speeds are concerned, is inferior to other processes for producing nonwoven webs. By way of example, a method that has been used to blend a mixture of long and short fibers into a non-woven web of randomly oriented fibers involved the step of introducing a mixture of preopened long and short fibers to a single lickerin where the mixture of long and short fibers is individualized. The individual fibers, but still in admixture, are introduced into an air stream and conveyed to a condenser where they were formed into a web. This method has a significant disadvantage in that in order to prevent degradation of the long fibers, it is necessary to operate the lickerin at optimum speed for the long fibers, which is much below that which is optimum for short fibers. This necessary compromise seriously limited the rate at which the fibers could be processed through this system and this economic disadvantage militates against its use. Also, this method is capable of producing only a single type of web, i.e., a web comprised of a homogeneous blend of long and short fibers.
Another prior art apparatus used to make a non-woven web that is intended to be a homogeneous mixture of randomly oriented long and short fibers includes the use of a milling device, such as a hammer mill, to individualize the short fibers and a lickerin to individualize the long fibers. The individualized short fibers are entrained in an air stream leading to a mixing zone into which the long fibers are introduced, where the fibers are intermixed. The mixture of fibers is deposited on a condenser to form a web of a random mixture of long and short fibers. In these webs, the intermixed fibers are not completely homogeneously blended; in fact, in such webs, there is more or less of a stratification of the fibers in layers, with the long fibers predominating on one side of the web and the short fibers predominating on the other side. A particular disadvantage of this apparatus was that the hammer mill did not completely individualize the wood pulp fibers and, in consequence, clumps of fibers and/or "salt" resulted. Also, only a single type of web can be produced by this approach.
Langdon U.S. Pat. No. 3,512,218, granted May 19, 1970, and Wood U.S. Pat. No. 3,535,187, granted October 20, 1970, disclose apparatus for producing layered, nonwoven webs, wherein the layers are apparently separated by a thin interface of blended fibers from each layer.
A recent development in this field of air-laying webs has overcomme a number of the aforementioned problems in the apparatus previously used and makes possible production of a nonwoven web of a homogeneous mixture of long and short fibers, free from consequential amounts of clumps and "salt". The apparatus and method of this development are described and claimed in a commonly owned United States application Ser. No. 108,547, filed Jan. 21, 1971, now U.S. Pat. No. 3,772,739, in the name of Ernest G. Lovgren.
In the Lovgren apparatus and process, long and short fibers to be blended are individualized separately and simultaneously by separate high speed lickerins, one for each type of fiber, that are operated at speeds optimum for the specific fibers acted upon. For example, in the case of pulpboard, the lickerin is operated in the order of 6,000 rpm to individualize the wood pulp fibers, and the long fibers, the staple length fibers, for example, rayon, are individualized by the lickerin acting on these fibers, operated at a speed in the order of 2,400 rpm. At a speed of 6,000 rpm, rayon fibers are damaged.
In the Lovgren apparatus, individualized fibers are doffed from their respective lickerins by separate air streams. The fibers are entrained in the separate air streams and the air streams are subsequently intermixed in a mixing zone to homogeneously blend the fibers entrained therein. The homogeneous blend of fibers is then deposited in random fashion on a condenser disposed in proximity to the mixing zone. The air streams generated by the high speed operation of the lickerins and by a suction fan located in the condenser, which acts to draw air past the lickerins, convey the fibers to the condenser.
While the Lovgren apparatus represents a substantial advance in the art, the apparatus has limitations in that it does not lend itself for use in making a wide variety of webs.
In accordance with a still further recent improvement, as described and claimed in a commonly owned United States application Ser. No. 108,545 filed in the name of Allan P. Farrington on Jan. 21, 1971, now U.S. Pat. No. 3,740,797, flexible process and apparatus are described for producing a wider variety of nonwoven, air-laid isotropic webs made up of a substantially uniform mixture of long and short fibers, or of two different kinds of long or short fibers. In accordance with the Farrington process, the following types of webs can be produced: (1) a web comprised of a homogeneous blend of fibers from two different fiber sources, (2) a web having outer layers comprised of fibers from two different fiber sources and an intermediate layer that is a blend of the fibers from each source, and (3) a web of two layers of fibers from each fiber source, with the layers being interlaced only at the region of their interface.
In yet another recent development, a still further improvement is disclosed for not only producing webs having greater uniformity, but also non-laminated webs having different properties at their opposite faces. Two similar webs [(2) and (3)] have been produced by the Farrington invention, as summarized in the preceding paragraph, but such webs do not obtain the different properties by a blend of fibers at the opposite faces. In accordance with the teachings in commonly owned United States application Ser. No. 108,546 filed in the name of Angelo P. Ruffo and Prashant K Goyal on Jan. 21, 1971, now U.S. Pat. No. 3,768,118, a web is produced from two different types of fibers at a given overall concentration, with the concentration of each fiber type being increased above the overall concentration at opposite faces, and with the concentration of each fiber type gradually decreasing away from the face at which the overall concentration is increased to the opposite side of the web. Ruffo et al also discovered that by providing an air-to-fiber volume ratio substantially in excess of those provided in the Lovgren and Farrington applications, i.e. in the range of about 12,000:1 to 275,000:1 in the combined air stream, extremely uniform webs can be produced at high production speeds up to 550 feet per minute or greater. These webs were of the type described above in this paragraph, or the type described in the preceeding paragraph.
While the Lovgren, Farrington and Ruffo et al applications all disclosed significant advances in the art, there remains a need for an apparatus and process for producing a wider variety of high quality webs at greater throughputs, and this need is satisfied by the process and apparatus of the present invention. Since the present invention is related to the inventions disclosed in the Lovgren, Farrington and Ruffo et al applications, the disclosures thereof are expressly incorporated herein by this reference to the extent that they are not inconsistent with the express teachings thereof.